In a conventional radio communication system, radio base stations (hereinafter, merely referred to as “base stations”) are systematically disposed to cover individual service areas. If multiple mobile terminals are located in a single service area, the mobile terminals share the radio resources to communicate with the base station. The service areas are called macrocells.
In recent years, some homes and offices have considered installation of base stations accessible only from/to specified mobile terminals at appropriate sites.
The Long Term Evolution (LTE) Release 8 standard, for example, involves femtocells each covering a relatively small area of several meters to tens of meters in addition to the macrocells.
A standardized example of the femtocell is a CSG cell accessible only from mobile terminals belonging to a group of preregistered mobile terminals (closed subscriber group; CSG).
As described above, a mobile terminal not belonging to a CSG cannot access the CSG cell. If such a mobile terminal could access the CSG cell, the convenience of the system would increase. For example, the connection to a CSG cell may provide higher communication quality than that of a macrocell in an area around the macrocell, and lead to effective use of the radio resources of the base station.
The LTE Release 9 standard involves hybrid cells accessible from/to not only mobile terminals belonging to the CSG but also mobile terminals not belonging to the CSG.
Although mobile terminals not belonging to the CSG are disadvantageous in terms of the quality of service (QoS) control and services in the hybrid cell, they can access the CSG cell, like a macrocell.
The CSG cell and the hybrid cell each transmit notification information containing a unique identifier (CSGID; csg-Identity) to all the mobile terminals therearound, to notify the mobile terminals that the cell is a CSG cell.
The mobile terminals determine whether the received notification information is transmitted from a CSG cell or hybrid cell on the basis of the unique identifier.
In specific, the mobile terminals determine whether the base station is a hybrid cell or CSG cell on the basis of csg-Indication, which is a Boolean parameter contained in System Information Block Type 1 (SIB1) in the notification information transmitted from the base station.
FIG. 1 illustrates example information contained in notification information SIB1 transmitted from a base station.
Each mobile terminal stores a list of CSGIDs (CSG white list) corresponding to CSG cells and hybrid cells accessible to the mobile terminal in a storage built in the mobile terminal, for example.
The mobile terminal compares a CSGID contained in notification information from a neighboring CSG cell or hybrid cell with its CSG white list to determine whether the mobile terminal belongs to the CSG of the CSG cell or hybrid cell.
If a mobile terminal located in a macrocell enters the coverage of a CSG cell or hybrid cell in a radio resource control connected (RRC CONNECTED) mode to the macrocell, the system executes a handover to the CSG cell or hybrid cell according to the process illustrated in FIG. 2.
FIG. 2 illustrates the process of a handover of a mobile terminal in an RRC CONNECTED mode to a CSG cell or hybrid cell.
A macrocell having a neighboring CSG cell or hybrid cell transmits an RRC Reconfiguration containing information on the neighboring cell (Report Proximity Config) to a mobile terminal connected to the macrocell (Step S1001).
The mobile terminal receives the RRC Reconfiguration and reads the CSGID of the neighboring cell on the basis of notification information from the neighboring cell.
The mobile terminal, which reads the CSGID of the neighboring cell on the basis of the notification information, refers to its CSG white list. If the mobile terminal determines the presence of a neighboring CSG cell or hybrid cell accessible to the mobile terminal, the terminal transmits a Proximity Indication containing a message (entering) indicating the determined results to the macrocell (Step S1002).
The macrocell receives the Proximity Indication; and transmits an RRC Reconfiguration containing Measurement Config, which is required for the mobile terminal to measure cell information on the CSG cell or hybrid cell, to the mobile terminal (Step S1003).
The mobile terminal receives the RRC Reconfiguration. If any event leading to a handover occurs, the mobile terminal reads the physical cell ID (PCI) of the CSG cell or hybrid cell on the basis of the notification information therefrom, and transmits a Measurement Report containing the PCI to the macrocell (Step S1004).
The macrocell receives the Measurement Report. If the macrocell determines that the reported PCI belongs to the CSG cell or hybrid cell, the macrocell transmits an RRC Reconfiguration containing a System Information (SI) Request to the mobile terminal (Step S1005).
The mobile terminal receives the RRC Reconfiguration, and reads the SI on the notification information from the CSG cell or hybrid cell (Step S1006).
The mobile terminal, which reads the SI on the notification information, transmits a Measurement Report, which contains the cell global ID (CGI), tracking area ID (TAI), and CSGID of the CSG cell or hybrid cell, to the macrocell (Step S1007).
The macrocell receives the Measurement Report, transmits a handover request to a mobility management entity (MME) (Step S1008), and executes a handover procedure (Step S1009). This process is defined in Non Patent Literature 1 below, for example.
In contrast, if a mobile terminal trying to connect to a cell is in a radio resource control idle (RRC IDLE) mode, the mobile terminal performs an initial access, one of the random access procedures, according to the process illustrated in FIG. 3, regardless of whether the cell is a CSG cell.
Before the start of communication between a base station and a mobile terminal, a radio communication system prepares a channel used for the start of transmission from the mobile terminal to the base station. In a 3GPP standard, the channel is called random access channel (RACH), and the process of starting communication through the RACH is called random access (RA).
In specific, upon the start of communication, the base station and the mobile terminal perform a communication process called random access procedure to establish a connection between the base station and the mobile terminal. In other words, the base station and the mobile terminal establish communication therebetween. The first signal that the mobile terminal transmits to the base station in the random access procedure is called Random Access (RA) Preamble.
FIG. 3 illustrates the process of an initial access performed by a mobile terminal in an RRC IDLE mode.
Upon trying to connect to a cell, the mobile terminal reads notification information transmitted from the cell.
The mobile terminal then generates an RA Preamble on the basis of prach-Config contained in the read notification information, and transmits the RA Preamble to the cell (Step S1011).
The cell receives the RA Preamble from the mobile terminal. If the cell accepts the mobile terminal, the cell transmits an RA Response to the mobile terminal (Step S1012).
The mobile terminal receives the RA Response from the cell, and transmits an RRC Connection Request to establish a radio resource control connection (RRC connection) to the cell (Step S1013). The RRC Connection Request is a message, from a mobile terminal trying to connect to a base station, for requesting the base station to allocate the radio resources.
The cell receives the RRC Connection Request from the mobile terminal, and transmits an RRC Connection Setup to the mobile terminal (Step S1014).
The mobile terminal receives the RRC Connection Setup from the cell, and transmits an RRC Connection Setup Complete to the base station to establish a radio resource control connection (Step S1015).
The base station receives the RRC Connection Setup Complete from the mobile terminal, and transmits an Initial User Equipment (UE) message to a core network including a verifier, such as a home subscriber server (HSS) (Step S1016). This process is defined in Non Patent Literature 2 below, for example.
The RA Preamble is a message, from a mobile terminal trying to connect to a base station, for notifying the base station of the presence of the mobile terminal within the coverage of the base station, as described above. Upon receiving the RA Preamble, the base station is informed only of the presence of a mobile terminal trying to connect to the base station, and cannot obtain information on the mobile terminal. In other words, the base station cannot determine whether the mobile terminal belongs to the CSG of the base station.
If a mobile terminal belonging to the CSG enters the coverage of a hybrid cell in the radio resource control connected mode, and if any event leading to a handover occurs, the mobile terminal and a cell perform a handover to the hybrid cell according to the above-explained process.
In this case, a base station corresponding to the hybrid cell is required to preferentially accept mobile terminals belonging to the CSG and prioritize the mobile terminals in operations such as QoS control, in cooperation with an apparatus such as MME of a core network, even if the radio resources in the hybrid cell have already been occupied (congested).
If a mobile terminal belonging to the CSG in the radio resource control idle mode enters the coverage of the hybrid cell and tries to activate the random access procedure for an initial access, the hybrid cell in the congestion state rejects the initial access from the mobile terminal regardless of congestion caused by the connection with another mobile terminal not belonging to the CSG, for example.
In other words, the mobile terminal belonging to the CSG, which should be prioritized, is prevented from performing an initial access by the other mobile terminal not belonging to the CSG.
In view of this problem, a conventional technique limits random access procedures performed by mobile terminals not belonging to the CSG (Patent Literature 1 below), for example.    Japanese Laid-open Patent Publication No. 2011-041281    Non-Patent Literature 1: 3GPP, TS36.300, V9.5.0 (2010-09), Subclause10.5.1.2    Non-Patent Literature 2: 3GPP, TS36.213, V9.3.0 (2010-09), Clause6